I look at the estimates but that doesn't really affect what I am willing to bid. The factors I take into consideration are: 1. How badly do I want the coin? For example, there is a 1913 sixpence in the current Heritage auction. Nice coin but I already have one that is as good. So I don't want it really badly. 2. How nice is the coin? Very subjective but is it desirable? There are some coins with a high grading that don't always look very nice. 3. What is the book value for the coin. Am I willing to bid over or under book value? 4. What have similar coins sold for in recent auctions. Again, for example, a 1913 UNC sixpence sold for 40 pounds not long ago. So that sets my market value. 5. How likely is it that a similar coin will come up for auction soon? Well, there are at least three in this London Coins auction alone so if I didn't get this one there will probably be plenty of subsequent opportunities. 6. Is it likely to be in my price range. For an expensive coin that I cannot really afford I might stick a low bid in just in case I get lucky. So, based on the above, for the 1913 sixpence, I set the maximum I am willing to pay at around 50 pounds (plus buyers premium). As it happens, that is bang in the middle of the Heritage estimate.